The present invention relates to melt processable thermoplastic polyolefin blends and a process for the preparation of a melt processable thermoplastic polyolefin blend. More particularly, the present invention relates to a process for the preparation of a polypropylene copolymer blend with EPDM and other ingredients using a twin screw extruder or Buss co-kneader. The blends of this invention exhibit very high impact strength at moderate concentrations of EPDM and enable injection molding, thermoforming and other conventional techniques to be applied for making end products that demand high impact strength such as bumpers for cars/jeeps and other light automobile vehicles.
In the early 1970""s interesting opportunities arose in the automotive field for plastic materials having a specific requirement with respect to weight reduction, safety, production and maintenance costs. The first application was that of bumpers. Automobile bumpers were traditionally made of chromium plated metal and thus were heavy, expensive and too stiff. The use of polymer blends such as elastomer modified polypropylene in place of metal solved a majority of these problems. Elastomer modified isotactic polypropylene exhibits considerable impact strength, good dimensional stability, low weight, low production and maintenance costs.
Significant developments took place in Italy in respect of plastic automobile bumpers. The consumption of elastomer modified polypropylene expanded rapidly in Italian and then in other European markets. Other thermoplastic materials, particularly modified polycarbonate and modified polybutylene terephthalate were also considered for similar applications. However, elastomer modified polypropylene offered the same performance at a substantially lesser cost.
Plastic automobile bumpers entered the Indian market along with the Maruti car production in India. Initially, the bumpers were molded in India with an imported blend from Japan. JP 04,214,711 (1992) describes blends of thermoplastic polyolefins, polyurethanes from polyols, non-aromatic polyisocyanates and low molecular weight polyamines exhibiting appreciable impact strength and coatability properties. JP 03, 252, 436 (1991) claimed blends of ethylene-propylene block copolymer, ethylene-propylene rubber, non-crystalline nylon and modified polypropylene exhibiting excellent heat and impact resistance properties. JP 02,191,656 (1990) describes compounds of thermoplastic resins, rubbers, cross-linking monomers, and other additives possessing very high impact strength. JP 01,271,450 (1989) claims blends of ethylene-propylene block copolymer, ethylene-propylene rubber and talc exhibiting excellent impact strength and flexural modulus. U.S. Pat. No. 5,030,694 (1991) describes blends of ethylene-propylene copolymer with EPDM and organic peroxides exhibiting good melt flow, impact strength and flexural modulus. EP 435,247 (1991) illustrates blends of propylene polymer and multiphase propylene-vinyl monomer graft copolymer exhibiting good moldability, impact resistance and heat resistance properties. EP 373,834 (1990) describes compounds of hydrogenated diene-alkyl aromatic compound polymer, propylene polymer and polyoxyphenylene, polyamides, or thermoplastic polyesters possessing good impact strength.
In all the patents described above, the primary aim was to improve impact resistance. In an automobile bumper application, the primary concern is impact resistance (the item must remain undamaged when subjected to an impact of 4 kmph at xe2x88x9223xc2x0 C., according to European regulations). In addition, higher stiffness, better aesthetics and environmental resistance are also important considerations. Further, since the bumper is to be assembled with metal parts, low linear thermal expansion coefficient is also required. Moreover, given that wide and thin shape of the bumper mold, enhanced flow at the molding temperature is also required.
The present invention takes into account all the above considerations in mind. The blends prepared in the invention can also be used for applications other than car bumpers where similar properties are required such as for molded luggage, furniture, automotive components and body panels and the like.
It is an object of this invention to provide blends of polypropylene copolymer with EPDM and other additives that allow injection molding, compression molding, thermoforming and other conventional techniques to be applied for making end products such as car bumpers that require, inter alia, high impact resistance.
It is another object of the invention to provide a process for the preparation of blends suitable for making car bumpers by injection molding.
It is another object of the invention to provide an improved process for the preparation of a blend with polypropylene copolymer, EPDM and other additives that exhibits suitable flow properties as well as mechanical properties (such as melt flow index: 10-15 g/10 minutes; flexural modulus: 8000-15000 kg/cm2; (notched Izod impact strength: 50-60 kg.cm/cm; and heat deflection temperature: 60-105xc2x0 C.) necessary for injection molding of car bumpers as well as their property requirements.
It is another object of the present invention to provide an improved process for the preparation of blends for use in manufacture of car bumper with moderate concentrations of EPDM, filler, carbon black, and an organic peroxide using a twin screw extruder or a Buss co-kneader.
The present invention provides polypropylene copolymer blends with EPDM with or without a filler, carbon black, and an organic peroxide as well as a process for the preparation thereof by extruding the components in a twin screw extruder or a Buss co-kneader all together or in separate batches, while for example, the twin screw temperature is maintained in the range of 180 to 250xc2x0 C. and the screws are rotated at a speed of 50 to 100 rpm.
Accordingly, the present invention relates to melt processable thermoplastic polyolefin blends comprising an intimate mixture of ethylene-propylene diene monomer (EPDM) in an amount of 5 to 25% by wt and a base block copolymer in an amount of 95 to 55% by wt, the balance if any comprising one or more conventional additives such as herein described, the polyolefin blend having a melt flow index in the range of 10 to 15 g/10 minutes when tested according to ASTM D1238.
The present invention also provides a process for the preparation of a melt processable thermoplastic polyolefin blend comprising, blending a base-block copolymer in an amount of 55 to 95% by wt of the total blend and EPDM in an amount of 25 to 5% by wt of the total blend, the balance if any comprising one or more of conventional additives, fillers, anti-oxidants, carbon black, melt flow enhancing additives.
In one embodiment of the invention, the blend has one or two melting peaks in the range of 160 to 170xc2x0 C. when tested in a differential scanning calorimeter at a uniform heating rate of 10xc2x0 C./minute in nitrogen environment.
In another embodiment of the invention, the ethylene content in EPDM is in the range of 55 to 65 wt %.
In yet another embodiment of the invention, the base block copolymer is a copolymer of ethylene and propylene.
In a further embodiment of the invention, an organic peroxide is added to the blend during blending operation in order to improve the melt flow index thereof.
In a further embodiment of the invention, the organic peroxide is selected from the group consisting of dicumyl peroxide, 2,5-dimethyl-2,5-di(tert.butyl peroxy)hexane and ditertiary butyl peroxide in the range of 0 to 2 wt %.
In yet another embodiment of the invention, carbon black is added in an amount of 0 to 20, preferably 12 wt % as a conventional additive.
In a further embodiment of the invention, conventional additive comprises of natural filler in an amount of 0 to 20 wt % and is selected from the group consisting of talc, calcium carbonate and mica.
In one embodiment of the invention, the polypropylene copolymer has a melt flow index in the range of 1 to 4 g/10 minutes; when tested at 230xc2x0 C. at 2.16 kg. load (according to ASTM D1238); and ethylene-propylene diene monomer (EPDM) with ethylene content in the range of 55 to 65 wt % possessing specific gravity 0.86 to 0.90; and Mooney viscosity in the range of 36 to 77 [ML(1+4)125xc2x0 C.]; with or without carbon black incorporated in an amount of 0 to 12 wt %.
In another embodiment of the invention, the blends comprise EPDM as an ingredient in a concentration of 5to 25 wt %.
In another embodiment of the invention, the blends possess density in the range of 0.92 to 0.96 g/cc while their base polymer polypropylene copolymer has 0.90 g/cc density when tested according to ASTM D792.
In another embodiment of the invention, the blends exhibit melt flow rate in the range of 10 to 15 g/10 minutes when tested according to ASTM 1238.
In yet another embodiment of the invention, the blends exhibit an Izod impact strength (notched specimens) in the range of 50 to 60 kg.cm/cm when tested on injection molded specimens of 3.2 mm thickness (cut from mid portion of the injection molded tensile bars of Type-I according to ASTM D638) according to ASTM D256.
In yet another embodiment of the invention, the polyolefin blends exhibit tensile strength in the range of 170 to 250 kg/cm2, when tested according to ASTM D638 method.
In another embodiment of the invention, the blends possess flexural modulus in the range of 8000 to 15000 kg/cm2, when tested according to ASTM D790.
In a further embodiment of the invention, the blends show heat deflection temperature in the range of 55 to 65xc2x0 C. with 18.2 kgf/cm2 stress; or 60 to 105xc2x0 C. with 4.6 kgf/cm2 stress according to ASTM D648.
This invention was carried out using a polyolefin polymer such as a polypropylene-ethylene block copolymer obtained in the form of granules after adequately adding stabilizers and antioxidants after polymerisation in the plant. The granules are dried at 80xc2x15xc2x0 C. for a period of two hours, preferably in an oven with air circulation facility. Ethylene-propylene diene monomer (EPDM) elastomer obtained in the form of granules was also dried separately in an oven at a preferred temperature of 80xc2x15xc2x0 C. for a period of at least two hours. Similarly, a natural filler selected from the group consisting of mica, talc and calcium carbonate, preferably with a particle size in the range of 10 to 20 microns and carbon black were also dried at the above mentioned temperature and time.
The objective of melt blending is to break the elastomer (EPDM) into as fine particles as possible and to disperse them uniformly in the polypropylene block copolymer matrix. This is achieved by means of a twin screw extruder or a Buss co-kneader with a specially designed screw profile that facilitates intimate mixing of the ingredients.
A typical most preferred blend in accordance with the present invention is prepared as follows with the amount of each ingredient being as indicated below:
The above ingredients are tumble mixed. A co-rotating twin screw extruder with a screw profile that would enhance intimate mixing of the ingredients was used later. Preferably, the extruder temperature is maintained in the range of 180 to 250xc2x0 C.; screw speed is preferably 50 to 100 rpm and the residence time is preferably 0.5 to 5.0 minutes. The extrudate is dipped in circulating cold water and later chopped into granules of length of 3 to 4 mm.
The blends may also preferably include an organic peroxide selected from the group consisting of dicumyl peroxide, 2,5-dimethyl-2, 5-di(tert.butyl peroxy)hexane and ditertiary butyl peroxide in the range of 0 to 2 wt % so as to selectively cut down the long chains of the base polymer so that the melt flow index of the product blend is enhanced without significant deterioration of the mechanical properties thereof.
The dried extrudate granules are injection molded into ASTM standard test specimens for the evaluation of properties such as tensile, flexural, Izod impact, heat deflection temperature and the like. Dry granules are also used to measure melt flow index, crystallisation kinetics (usign differential scanning calorimeter), filler content (using thermogravimetric analyser), dispersion of EPDM and filler in the PPBC matrix (using polarized optical microscope), and the like.
Injection molding is preferably carried out using a computer controlled injection molding machine with a temperature profile (with four heating zones) in the range of 180 to 230xc2x0 C.; injection pressure (applied in six stages) of 15 to 125 Kg/Cm2, injection time (in six stages) of 2.5 to 5.0 sec with screw speed (in two stages) in the range of 80 to 100 rpm. Standard test specimens thus obtained were used for evaluating various performance properties of the blends following the ASTM standard test methods.
The principal performance property required for a bumper application is impact resistance. The elastomer concentration as well as its particle size in the blend is found to play a prominent role in controlling the mechanisms that affect the toughness of the matrix. The phenomenon of craze initiation in PPBC is greatly dependant on the EPDM particle size. It is recognised, in general, that there appears to be an optimum size of rubber particles for toughening the rubber modified polymers. In the case of notched Izod impact strength, reduction in rubber particle size has gradually increased the strength. With reduction in EPDM particle size, the number of particles has increased and the inter-particulate distance has reduced. The multiple crazing in the blend gets further enhanced with the increased number of rubber particles and also reduced particle size. The shear yielding, which was observed in scanning electron microscope on fracture surfaces, also gets influenced with change in particle size resulting in increased impact strength. Further, blending of PPBC with EPDM does not influence the crystalline structure significantly, as observed on hot-stage optical microscope and DSC cooling thermograms. However, the xcex2-content of the PPBC increases due to the reduction in the EPDM particle size. As a result of thermal and mechano-chemical oxidation reactions in the presence of sulfur, the EPDM gets cross-linked during blending. Cross-linked EPDM gets dispersed in the matrix as spherical particles, contributing to the rise in impact resistance of the blends. The presence of fillers imparts high flexural modulus and heat deflection temperature, whereas carbon black offers resistance to ultra violet radiation. Other additives are used to provide high flow property as well as long term stability to the blend.
The present invention will now be described with reference to the following examples, which along with the foregoing preferred embodiments are illustrative and should not be construed as limiting the scope of the invention. Various alterations and modifications will be apparent to those skilled in the art and fall within the scope of the invention herein.